Downy mildew caused by the fungus Peronospora destructor (Pd) is practically world-wide in distribution. The pathogen attacks various kinds of onions, but is especially destructive to the common onion Allium cepa. The damage caused by the fungus is described in Mukerji (ref 5). Spontaneous epidemics may occur in the field, if conditions are favourable for the disease. The symptoms are yellowing of the leaves and grey sporulation.
From an economical point of view, downy mildew is one of the major fungal diseases threatening the cultivation of onions and shallots (Allium cepa L.), almost in all onion-growing regions in the world.
It is assumed that no complete natural resistance to downy mildew is present in Allium cepa and Allium fistulosum. However, a complete resistance to downy mildew was found in the wild Allium roylei Stearn. Inspired by a certain morphological resemblance between A. roylei and A. cepa it has therefore been proposed to use A. roylei as an introgression partner for A. cepa. In the publication by van der Meer and de Vries (ref 1) preliminary results are reported concerning such a hybrid between A. roylei and A. cepa, as well as results concerning the backcross of the interspecific hybrid to A. cepa. It has been observed that said interspecific hybrid is male and female fertile (Kofoet et al, ref 2). It has also been reported that a downy mildew resistance can be dominantly inherited in the BC1 (first backcross) progeny of this hybrid to onion.
The locus in the cepa genome, where the introgressed sequences responsible for the resistance are found, has been termed “Pd resistance locus”. By extension, “Pd resistance locus” also designates the sequences themselves.
However, segregation for downy mildew resistance among BC1 and F2 progenies from the F1 between A. roylei and A. cepa is observed (de Vries et al, ref 3) and no breeding onion varieties resistant to downy mildew have ever been obtained, more than 10 years after the first resistant hybrid was obtained.
There is thus great interest from an agronomical and economical point of view for plants of Allium cepa and Allium fistulosum species, which are resistant to downy mildew and which are still 100% resistant after self-pollination i.e. there is no segregation of the resistance characteristic. These plants are particularly valuable as they may indeed be crossed with a susceptible line, giving hybrids which are also resistant to downy mildew of onion.
The invention lies first in the observation that all available resistant material of the Allium cepa and Allium fistulosum species was in fact heterozygous for the Pd resistance sequences and that no homozygous resistant plants have been obtained or disclosed in a reproducible manner up to now.
The present inventors elucidated the reason for which no homozygously resistant plant could be obtained and then succeeded in obtaining such Allium plants, which gives rise to progeny after self-pollination which is also 100% resistant (i.e. a homozygously resistant plant).
Several hypotheses could have explained the striking finding that no homozygous plants were available: translocation, recombination, hybrid preference selection, gene silencing and pleiotropic effects, linkage drag, etcetera . . . .
The present inventors have determined that the introgression fragment of Allium roylei which is present in the cepa hybrid and confers resistance to downy mildew comprises also a sequence designated “lethal factor”, whose presence on both homologues of chromosome in cepa is lethal to the plant. For a resistant cepa plant to exist and grow, the introgression fragment, conferring resistance and containing the lethal factor, is thus necessarily present on a single chromosome homologue, explaining the absence of obtained homozygously resistant plants by the predecessors. This sequence responsible for lethality is present in the vicinity of the Pd resistance locus.
Having identified that the sequence conferring resistance is linked to a lethal sequence, the present inventors have succeeded, for the first time, in physically separating the sequence conferring resistance from the sequence responsible for lethality when present on both chromosomes.
This step of separation, which can be brought about by a recombination event, is the key point of the present invention. Indeed it could not be foreseen that the sequences conferring resistance could be separated from the lethal factor. First of all, it was not known whether said factor was a sequence which in itself was lethal or whether lethality arose as a result of the replacement, by sequences present on an introgression fragment, of endogenous essential sequences. Indeed, it is possible that the lethality is in fact a knock out of essential gene(s) on the corresponding cepa-fragment, which are not compensated by the fragment introduced from A. roylei. Therefore, different scenarios were possible to explain lethality:                the resistance sequences might have constituted in fact the lethal factor, because they replace endogenous critical sequences. They could thus never be present on both chromosomes without causing lethality.        the resistance sequences and the lethal sequences might have been extremely near to each other on the chromosome, or even overlapping. In this situation, the probability of obtaining a recombination event separating both is negligible.        the sequences constituting the lethal factor might have been necessary for the functioning of the resistance. In this condition, separating both would lead to the loss of the capacity for resistance.        
In this context, the present inventors have unexpectedly succeeded in separating the sequences conferring the resistance phenotype from the sequence whose presence on both homologues is lethal.
The inventors have also found that the already released resistant A. cepa possess both the Pd resistance locus (which is dominant) and the lethal factor (which is recessively inherited) on the same introgression fragment. As the Pd resistance locus is dominant, the phenotype of the plant is “resistant” and the recessive lethal factor is not observed. However, progeny of such plants when crossed with susceptible plants will segregate and thus cannot be used commercially. Indeed, commercial onions and shallots are generally varieties, thus plant material, whether lines or hybrids, with a phenotype which has to be uniform; segregating traits of interest cannot be envisaged for commercial plants.
The present invention thus provides plants of the Allium cepa and Allium fistulosum species which are resistant to downy mildew of onion caused by Peronospora destructor (Pd) due to the presence in their genome of a Pd resistance locus, and said plants are homozygously resistant plants, i.e. plants which give a progeny after selling which is also 100% resistant.
The present invention also provides resistant plants which can be obtained by crossing the above-mentioned resistant plants of the Allium cepa or Allium fistulosum species with a plant also of the Allium cepa or Allium fistulosum species which is susceptible to downy mildew.
The present invention also provides processes for obtaining said plants.